The federal government of the United States has promulgated test methods in 40 CFR Part 60, Appendix A for determining stack gas velocity and volumetric flow rate. If one knows the flow rate and has another monitor which measures the concentration of pollutants in a selected volume of fluid one can calculate the quantity of pollutants emitted over any selected time period. Accordingly, the test methods have been used in various ways, including the verification of the performance of continuous emission monitoring equipment required by other rules.
The United States has additional regulatory requirements in the form of 40 CFR, Parts 72 through 75 (acid rain reduction), which utilize the Appendix A methods. Some recent regulations now require many electric utilities to continuously measure emissions of specified pollutants on a mass per unit time basis. Adoption of these rules has put a new importance on the errors in both the continuous monitor and in the referenced test methods. The new regulations establish monetary value in the form of trading credits to a ton of SO.sub.2 emissions. The value of such emissions is such that for large utilities as much as $1,000,000 per percent error in measured emissions may result.
The regulations require that calibration tests be periodically performed to assure that the continued flow monitor equipment is working properly. One type of common flow monitor used EPA compliance relies upon differential pressure (.DELTA.P) measurements using a Type-S pitot tube connected to a pressure transducer. Prior implementations of .DELTA.P devices for CFM applications have, at most, employed a quasi-calibration technique wherein the pressure transducer is isolated from the pitot and is subjected to one or more known pressures. This procedure tests only the pressure transducer and is unable to detect pluggage, erosion, corrosion and leaks that could substantially alter the stack reading. Indeed, it has been shown that, on systems that use long lengths of pressure tubing between the pitot and the transducer, a leak of magnitude that can appreciably alter the stack ft/sec reading is not only undetectable during the daily calibration check, but is not even detectable using a manual leak check procedure.
There is a need for an accurate and convenient method and apparatus for performing calibration checks of pitot tube based continuous flow monitors.